#define _CRT_SECURE_NO_WARNINGS 1
#include<stdio.h>
#include<math.h>
int main(void)
{
	int N;
	double i,sum = 0;
	scanf("%d", &N);
	for (i = 1; i <= N; i++){
		sum = sum + sqrt(i);
	}
	printf("sum = %.2lf", sum);
	return 0;
}